This invention relates to techniques of etching a copper foil or copper plate, and more particularly to an etching solution or etchant for roughening a surface of a copper foil or copper plate so that the surface has acicular protrusions, a method for roughening a copper surface and a method for producing a printed wiring board in which a defect such as a pink ring or the like can be prevented from occurring and the number of manufacturing steps can be reduced.
A multi-layer printed wiring board has been conventionally made by laminating an inner layer material, an outer layer material and prepregs on each other. A copper-clad laminate on which a copper foil is laminated is used as the inner layer material and the like. In general, manufacturing of such a multi-layer printed wiring board is carried out in such a manner that the copper-clad laminate is subjected to a pretreatment such as a rust prevention and the like (preliminary treatment prior to circuit formation); the copper-clad laminate is then subjected to patterning and the like to form a copper conductive pattern layer (circuit formation); a roughening treatment is conducted to roughen a surface of the copper conductive pattern layer which is an inner layer material; an outer layer material such as resin, a film, ink or the like is laminated on the roughened surface of the inner layer material or copper conductive pattern layer to form a laminate (laminating); and the laminate is formed with through-holes and then subjected to electroplating.
As shown in FIG. 1, the preliminary treatment prior to the circuit formation generally includes removing a rust preventive film, washing with water, micro-etching, washing with water, rust preventing, washing with water and drying. The surface roughening as described above is executed by any one of a first process of forming a layer of copper oxide such as cuprous oxide or cupric oxide on the surface of the copper conductive pattern layer (blackening process), a second process of reducing such a copper oxide layer as described above to metallic copper using a reducing agent while keeping the configuration of the oxide layer (reducing process) and a third process of forming a metallic copper layer of coarse particles on a copper conductive pattern layer by electroless plating of copper (electroless copper plating process).
When the first process is employed, the following treatments are carried out sequentially: alkali degreasing, washing with water, acid degreasing, washing with water, micro-etching, washing with water, predipping, blackening, washing with water and drying. When the second process is employed, the following treatments are carried out: alkali degreasing, washing with water, acid degreasing, washing with water, micro-etching, washing with water, predipping, blackening, washing with water, reducing, washing with water, rust preventing, washing with water and drying. Further, when the third process is employed, alkali degreasing, washing with water, acid degreasing, washing with water, micro-etching, washing with water, predipping, catalyst provision, washing with water, catalyst activation, washing with water, electroless copper plating, washing with water, washing with acid, washing with water, rust preventing, washing with water and drying (see FIG. 1).
Unfortunately, the first process encounters a serious problem. More particularly, copper oxide is generally dissolved in acid. Thus, when the copper oxide of the copper conductive pattern layer is exposed on an inner surface of the through-holes due to formation of the through-holes, dipping of the copper conductive pattern layer in an etching solution or etchant during the subsequent electroplating causes the copper oxide to react with sulfuric acid in the etchant, to thereby be dissolved in the form of copper sulfate in the etchant, resulting in a defect called a pink ring occurring on the conductive pattern layer.
The second process needs to carry out reduction of the copper oxide to metallic copper after formation of the oxide, leading to an increase in the number of steps in the process. Also, it has another disadvantage of causing an increase in manufacturing cost of a printed wiring board because a reducing agent for the reduction is expensive. The third process likewise increases the number of steps.
The present invention has been made in view of the foregoing disadvantages of the prior art.
Accordingly, it is an object of the present invention to provide an etching solution or etchant which is capable of providing a copper conductive pattern layer with a roughened surface exhibiting increased acid resistance.
It is another object of the present invention to provide a method for roughening a copper surface which is capable of firmly joining a copper conductive pattern and an outer layer material to each other to reduce the number of steps in manufacturing of a printed wiring board.
It is a further of the present invention to provide a method for producing a printed wiring board which is capable of preventing a defect such as a pink ring or the like from occurring on a conductive pattern layer.
In accordance with one aspect of the present invention, an etchant is provided. The etchant includes a main component containing at least one first compound selected from the group consisting of oxo acids represented by one of the following formulae:
XOm(OH)n and HnXOm+n
where X is a central atom, m is an integer of 0 or more and n is an integer of 1 or more and derivatives thereof and at least one second compound selected from the group consisting of peroxides and derivatives thereof; and an auxiliary component containing at least one azole and at least one halide.
In a preferred embodiment of the present invention, the integer m in the above-described formulae is 2 or more.
In a preferred embodiment of the present invention, the integer m+n in the above-described formulae is 4 or more.
The oxo acids suitable for use in the present invention typically include sulfuric acid (H2SO4). Also, the oxo acids may further include nitric acid (HNO3), boric acid (H3BO3), perchloric acid (HClO4), chloric acid (HClO3), phosphoric acid (H3PO4) and the like. The oxo acid derivatives may include 2-hydroxyethane-1-sulfonic acid (HOC2H4SO3H), methanesulfonic acid (CH3SO3H), aminosulfonic acid (NH2SO3H), hydroxybenzenesulfonic acid (HOC6H4SO3H), nitrobenzenesulfonic acid (NO2C6H4SO3H), p-aminobenzenesulfonic acid (NH2C6H4SO3H) and the like.
The concentration of an oxo acid or derivative thereof in the etchant of the present invention is selected in view of the degree of the resultant roughness of the copper surface. If the concentration of the oxo acid or derivative thereof is too low or too high, the copper surface cannot be provided with sufficient irregularities. For example, for sulfuric acid (H2SO4), the concentration thereof is preferably 40 to 300 g/l, and more preferably 65 to 200 g/l; for 2-hydroxyethane-1-sulfonic acid (HOC2H4SO3H), the concentration thereof is preferably 60 to 300 g/l, and more preferably 100 to 250 g/l; for aminosulfonic acid (NH2SO3H), the concentration thereof is preferably 45 to 160 g/l, and more preferably 60 to 150 g/l; and for methanesulfonic acid (CH3SO3H), the concentration thereof is preferably 60 to 300 g/l, and more preferably 60 to 180 g/l, respectively.
The peroxides may typically include hydrogen peroxide (H2O2), and the peroxide derivatives may include peroxo acids, peroxonates and the like. In particular, hydrogen peroxide, peroxomono acid or a salt thereof are preferably used as the peroxide or derivative thereof. The peroxo acids may include peroxomonosulfuric acid (H2SO5), peroxonitric acid (HNO4), peroxomonophosphoric acid (H3PO5), peroxochromic acid (H3CrO8), peroxoboric acid (HBO3, HBO4, HBO5) and the like. The peroxonates may include potassium peroxomonosulfate (K2SO5), potassium hydrogenperoxosulfate (KHSO5), potassium peroxonitrate (KNO4), sodium peroxomonophosphate (Na3PO5), sodium peroxochromate (Na3CrO8), sodium perborate (NaBO3, NaBO4, NaBO5) and the like.
The concentration of a peroxide or derivative thereof in the etchant of the present invention is a concentration which allows a suitable etching rate to be exhibited. If the concentration of the peroxide or derivative thereof is too low, the etching rate is too low to be of practical use; and if it exceeds a certain value, the etching rate is too high to control. For example, for hydrogen peroxide (H2O2), the concentration thereof is preferably 20 to 200 g/l, and more preferably 40 to 80 g/l; and for potassium peroxomonosulfate (K2SO5), the concentration thereof is preferably 60 to 300 g/l, and more preferably 120 to 250 g/l, respectively.
Also, the etchant includes an auxiliary component containing at least one azole and at least one halide. The azoles may include triazole, pyrrole, oxazole, thiazole and the like. The triazoles and derivatives thereof may include benzotriazole (BTA), 5-methylbenzotriazole and the like. The azole concentration in the etchant of the present invention is a concentration sufficient to provide a copper surface with irregularities. If the azole concentration is too low, sufficient etching capability cannot be exhibited; and if it exceeds a certain value, a degree of surface roughening does not vary. For example, the concentration of benzotriazole (BTA) used as the azole in the etchant is preferably 0.1 to 20 g/l, and more preferably 1 to 10 g/l.
The halides may typically include chlorides such as hydrochloric acid, hydrochloride and the like. The chlorides may include sodium chloride (NaCl), potassium chloride (KCl), stannous chloride (SnCl2) and the like. The halide concentration in the etchant of the present invention is a concentration sufficient to provide a copper surface with irregularities. If the halide concentration is too low, a degree of surface roughening is too low; and if it exceeds a certain value, a degree of surface roughening instead is decreased. When a chloride such as sodium chloride (NaCl), potassium chloride (KCl), stannous chloride (SnCl2) or the like is used as the halide in the etchant, the chloride concentration is selected so that the chlorine concentration in the etchant may be preferably 0.0006 to 1.21 g/l, and more preferably 0.006 to 0.182 g/l. For example, the concentration of sodium chloride (NaCl) used as the halide in the etchant is preferably 0.001 to 2 g/l, and more preferably 0.01 to 0.3 g/l.
In accordance with another aspect of the present invention, a method for roughening a copper surface is provided. The method comprises the step of subjecting the copper surface to etching using an etchant so that the copper surface is provided with acicular protrusions. The etchant includes a main component containing at least one first compound selected from the group consisting of oxo acids represented by one of the following formulae:
XOm(OH)n and HnXOm+n
wherein X is a central atom, m is an integer of 0 or more and n is an integer of 1 or more and derivatives thereof and at least one second compound selected from the group consisting of peroxides and derivatives thereof. Also, the etchant further includes an auxiliary component containing at least one azole and at least one halide.
In accordance with a further aspect of the present invention, a method for producing a printed wiring board using an inner layer material having an insulating layer on which a copper foil layer is laminated is provided. The method comprises the steps of roughening a surface of the copper foil layer of the inner layer material by an etchant, subjecting the roughened copper foil layer to patterning to form a copper conductive pattern layer, and then laminating an insulating outer layer material on a surface of the copper conductive pattern layer. The etchant includes a main component containing at least one first compound selected from the group consisting of oxo acids represented by one of the following formulae:
XOm(OH)n and HnXOm+n
wherein X is a central atom, m is an integer of 0 or more and n is an integer of 1 or more and derivatives thereof and at least one second compound selected from the group consisting of peroxides and derivatives thereof. The etchant further includes an auxiliary component containing at least one azole and at least one halide.
In a preferred embodiment of the present invention, a degreasing treatment and a predip treatment are conducted prior to the roughening step using the etchant.
The etchant of the present invention, when copper is dipped therein, permits the auxiliary agent to form a carrier layer on a surface of the copper. Also, it permits copper to be selectively dissolved in the form of copper ions from crystal defects on the copper surface, resulting in electrons being discharged. The discharged electrons are then fed to the peroxide through the carrier layer, leading to reduction of the peroxide and formation of water on an interface between the carrier layer and the etchant. Thus, a concentration gradient of copper ions occurs across the carrier layer in such a manner that the concentration is increased near the copper and reduced near the etchant, to thereby exhibit a function as a local cell. This would result in copper being selectively dissolved from the deepest or deepmost portion of the copper surface at which a concentration of copper ions is increased, leading to formation of irregularities on the copper surface.